Many probes are in use today to measure or monitor a function of the body to which the probe is attached or to control or influence a body state or condition. For example, the thermal environment of an infant incubator may be regulated by sensing the body temperature of the infant and developing a signal, representative of the infant's body temperature, for controlling the incubator heater. In such an application, it is important that intimate contact, between the skin and the probe contact surface which carries the temperature sensor, be established and maintained. Should the probe become dislodged, resulting in a total or partial loss of contact, the heater will respond to a temperature measurement other than the infant's body temperature.
One approach, currently in use, for detecting probe dislodgement involves using information directly from the function being monitored. A significant deviation in the measurement of a body function may be the result of a significant change in the body function itself or the result of the measuring probe being dislodged from the body. In either case, an indication of the condition should be developed. While such an approach may be adequate for certain applications to sense a complete dislodgement of the probe, a partial dislodgement may produce an inadequate deviation and go unnoticed. Also, in some applications, the function being monitored may not change so appreciably upon probe dislodgement as to produce an adequate indication. For example, in an incubator, the ambient temperature may be approximately equal to the temperature of the infant. Therefore, when a probe attached to the infant becomes dislodged and exposed to the thermal environment of the incubator, there may be very little, if any, immediate change in the output of the temperature sensor.
Another technique which has been suggested for detecting probe dislodgement involves sensing impedance changes due to changing contact conditions between the probe and the body. This approach has met with only limited acceptance. It has been found that various other factors besides loss of contact between the probe and the body affect the impedance. As a result, this technique does not provide adequate reliability.
United States patent application Ser. No. 075,253 filed on Sept. 13, 1979 by Benjamin L. Hochman and United States patent application Ser. No. 088,474 filed by Terence A. Torzala on Oct. 26, 1979 each disclose an optical technique for detecting probe dislodgement. The apparatus disclosed in the Hochman application and the apparatus disclosed in the Torzala application each sense a change in the level of radiation impinging upon the body contact surface of the probe when the probe is dislodged from the body. Although the techniques disclosed in the Hochman and Torzala applications are extremely useful in effectively detecting probe dislodgement, the particular apparatus disclosed in the Hochman and Torzala applications require the addition of optical components to the probe structure and special circuitry to protect against false alarms.